Proper functioning of the nervous system relies on the establishment of specific neural pathways, apparently through mechanisms that specify synaptic connections at some stage during ontogeny. As a step toward understanding these processes, this project will examine the functional and structural differentiation of two types of principal neurons in bullfrog (R. catesbiana) sympathetic ganglia during development, in vivo. Lumbar sympathetic ganglia of the frog have two identifiable classes of principal cells, B and c neurons, that are selectively innervated by two distinct populations of preganglionic axons, B and C fibers. Preliminary studies indicate that differences in axonal conduction velocity, used to distinguish between B and C neurons in the adult frog, are not expressed until after synaptogenesis. Recent analysis of acetylcholine current noise shows that, in the adult frog, the kinetic properties of synaptic channels of B neurons differ from those of C neurons. Furthermore, it appears that these neurons do not acquire chemosensitivity to acetylcholine until they are innervated by preganglionic axons. The appearance of these functional differences subsequent to synaptogenesis raises the question: is the specification of a particular sympathetic neuronal phenotype determined by interaction with corresponding type of preganglionic fiber? The specific aims of this proposal are to examine and compare differences in the development of B and C neurons according to synaptic channel properties, axonal conduction velocity, and synaptic structure at the light and electron microscopic levels. Finally, the role of the preganglionic fiber will be tested by examining whether or not the expression of a particular functional or structural property requires the presence of the corresponding type of preganglionic innervation.